1. Field of the Invention
This invention relates to a heat-shrinkable tube for use in forming a connection part for shielded polyolefin insulated electrical wires or cables.
2. Description of the Prior Art
Recently, cross-linked polyethylene insulated electrical wires and cables have tended to be used at much higher voltages, and accompanied with this tendency further improved reliability of connection parts for cross-linked polyethylene insulated electrical wires and cables is required. For this purpose, it has been heretofore proposed to use so-called molded connection parts, which are obtained by applying a reinforcing insulation layer made of rubber, plastics, etc., around a connection part of an electrical wire or cable and heat molding this assembly under pressure to cause the reinforcing insulation layer to form an integrated unit with the insulation layer of the cable.
On the other hand, there has also been a recent tendency that in the practical application of these molded connection parts of wires and cables, coverage of distances much longer than ever before are encountered and therefore it has become increasingly necessary to provide an insulated connection part in which a specific insulated portion, i.e., an insulated portion having a specific structure consisting of two shielding members (outer semiconductive members) combined with each other through an insulation material, is provided. Heretofore, conventional methods for forming molded connection parts for cross-linked polyethylene insulated electrical wires or cables have generally comprised applying a reinforcing layer around the insulated connection part by winding a tape made of polyethylene containing a cross-linking agent or cross-linked polyethylene, or alternatively pouring such cross-linking agent containing polyethylene or cross-linked polyethylene into a metal mold provided around the connection part, winding around the thus-provided reinforcing layer a tape of a semiconductive material, and then a tape of an insulating material, to thereby form a specific insulated portion. Since these conventional methods require heat molding of the connection part as a whole simultaneously after providing the semiconductor and the specific insulated portion, they may produce many defects. For example, the tip of the outer semiconductive layer in the specific insulated portion can be fluidized or deformed to form a sharp protrusion, and the interface between the reinforcing insulation layer and the connection part can undulate and lose smoothness such that the electrical wire or cable cannot provide satisfactory electrical properties.
In order to eliminate the above-described disadvantages, a method has heretofore been proposed which comprises grinding the surface of the reinforcing insulation layer after heat molding to smooth it, winding a tape of semiconductive material around a reinforcing layer on one conductor in a region extending from one end of the reinforcing layer to the center thereof and then winding a tape of an insulating material around the end of the wound tape of the semiconductive layer to thereby form a specific insulated portion, and winding a tape of semiconductive material around the other conductor. This method, however, is disadvantageous in that molding takes a rather long time.